Fine grinding of polymers in a vibrated bead mill

Three polymers, having various glass temperatures, have been ground in a sh aker bead mill. One is a friable polymer (poly(vinyl acetate), PVA) and two are thermoplastic polymers (polystyrene (PS) and polyethylene (PE)). These materials have been ground for various operating conditions (bead and poly mer loads, vibration frequency, temperature). It was found that, in the cas e of the friable polymer, the size reduction occurs mainly according to a c leavage mechanism. Nevertheless, in specific operating conditions, attritio n may happen. In the case of the thermoplastic polymers, different steps ap pear during the size reduction: abrasion, creation of cracks and fragmentat ion. Then, the fragmentation mechanism depends on the polymer nature. It is shown that it is necessary to work with a temperature below the glass temp erature of polymer. Generally, the rate of size reduction increases with a decrease of the bead load and of the powder load. In the case of PVA, the e volution of the size distributions has been predicted by two statistical la ws (the normal law at the beginning of the experiments and a combination of the normal and the log-normal laws at the end), whereas for PS, only log-n ormal laws have permitted to predict the evolution of the size distribution s. (C) 1999 Elsevier Science S.A. All rights reserved.